This invention relates to a method of testing a metal structural element, using precise measurements of eddy currents generated on the surface of the structural element being tested, to determine the useful life of the structural element without destroying it. Such eddy currents are induced in a mass of conducting material by a varying magnetic field.
Metal structural elements of a vast array of devices are routinely subjected to severe stresses under which they are designed to operate over the endurance life of the devices. Such stresses are caused by forces producing, or tending to produce deformation in a device or a portion of it; the stresses are measured by the force applied per unit area, for example as dynes per square centimeter (or pounds per square inch); the forces are typically axial torsional or bending. To increase their service life, metal structural elements of ferrous metals, aluminum, titanium and other metals which are susceptible to an increase in residual compressive stress at the surface when peened or shot-peened, are routinely shot-peened, which delays fatigue failure.
Fatigue refers to the failure of materials under the action of repeated stresses; it is responsible for a large proportion of the failures occurring in any one of a myriad structural parts of an aircraft, wheels of heavy duty trucks and rail cars, and a wide array of machine parts. But the expected service life of any of the structural parts is purely conjectural; the expected life is typically estimated from prior experiences with actual failures, or by destructive testing of an essentially identical device, such testing being carried out under what is believed to be the same spectrum of stresses to which such devices are expected to be subjected.
Typically, a series of fatigue tests are carried out on a number of specimens of a particular structural element at different stress levels, until each specimen fails; the stress endured by each specimen is then plotted against the number of cycles sustained. For steel structural elements, choosing lower and lower stresses, a value for stress may be found which will not produce failure even after a very large number of cycles. This stress value is termed the xe2x80x9cendurance limitxe2x80x9d and the diagram is referred to as a stress-cycle diagram or S-N diagram. In structural elements made of aluminum alloys, the build up of residual stress is more cumulative than in steel and less predictable. In the design and construction of devices where weight and cost are critical, designing a device to operate at stress values low enough to produce the endurance limit is not an option; the goal is the opposite, namely to design a device to operate at as high a stress value as will fall just short of the endurance limit.
For example, the landing gear of an aircraft is designed to operate for some predetermined period of time under preselected operating conditions. Failure of a critical strut in the landing gear under a chosen cyclical load, can be observed when the strut breaks. For the chosen strut, and every other structural element, there is a combination of peak load and number of cycles which provides a 50% failure point, that is, the point at which 50% of all the parts tested will have failed under those test conditions. In the field, an engineer does not know what peak load a particular part has endured, nor, typically, the number of cycles. Therefore the time when the part will fail during its normal operation is unpredictable.
Assuming one was to test a single strut, from a batch of many essentially identical struts which had been in similar service, until that strut failed, the test information might be used to predict the useful life of the remaining struts. Unfortunately, one cannot predict with reasonable certainty, the period after which a strut on the landing gear of an aircraft will fail after the aircraft is placed in service. Landing gear, typically of aluminum or titanium, is designed to withstand the forces generated by that aircraft not only while it is at rest, or while it is hurtling down a runway prior to take-off, but also when it lands. As is well-known, each landing is different from another, some, for example those on a pitching deck of an aircraft carrier, generating stresses an order of magnitude (ten times), or more, greater than those on a deck of the carrier on a calm sea. It therefore is imperative that the aircraft and its landing gear be removed from service well before its imminent fatigue failure. Knowing when to do so, until the discovery disclosed herein, has not been possible.
It should be recognized that, were it possible to identify precisely, the 50% failure point for a structural element which was still in service, the identification, in reality, would have been too late, because by definition, there was a 50% probability that the structural element would have already failed.
A structural element such as a strut of a ferrous metal or any other structural element of a device or machine may be checked by magnafluxing the element, which requires a large enough disturbance of the magnetic flux to allow the magnetic powder to gather in the vicinity of a non-uniformity, such as a crack. When this occurs the accumulation of residual stresses in the part has already reached or exceeded a xe2x80x9csafe-operationxe2x80x9d point where failure of the part is imminent, that is, the accumulation has progressed too far to allow the device to be operated safely. An accumulation of stresses past the xe2x80x9csafe operationxe2x80x9d point may also be observed in steel and aluminum devices with dye penetrants commercially available in xe2x80x9cspot checkxe2x80x9d kits.
Checking the structural element by X-ray provides information relating to a change in strain as evidenced by changes in a diffraction pattern from surface atoms, where there is displacement of atoms or distortion of grain structure, to a depth limited to less than 50 xcexcm (microns), typically less than 20 xcexcm, and often as little as 10 xcexcm. Seeing such variations provides no information as to how many stress cycles the structural element has endured, nor the magnitude of the strain. If, just before the X-ray measurements are made, the sample has relaxed its accumulated internal strains sufficiently so as xe2x80x9cto reportxe2x80x9d normal atomic spacing, the conclusion derived from such information, though an excellent method for determining the condition of the element at that particular time, would be misleading. Moreover, X-ray measurements are too costly and time consuming, therefore generally impractical.
The Problem: In the example of the landing gear just provided, over the course of several years, it is presently not possible to make an educated, economical appraisal of the condition of any of its structural elements at any time after it has been in service, and no method of determining how close to failure that element might be. If one could predict that a stressed component of any device would fail within a specified window of time, assuming operation of the device was continued, then, without otherwise interrupting operation of the device, that component could be taken out of service no later than, and preferably before the prediction indicated that the component had reached a xe2x80x9csafe-operationxe2x80x9d point, despite the component appearing to be in good condition. The problem is to find a method which allows one to make that prediction with reasonable accuracy.
U.S. Pat. No. 5,610,515 teaches a method of measuring eddy currents modified by residual stress in non-ferromagnetic metal objects, using certain circuit elements in an alternating current circuit the values of near-surface residual stress can be inferred. Generated eddy currents measured are modified by near-surface compression or tension resulting from working the surface for example, by shot-peening it, or by rolling material from which a structural element is fabricated. The term xe2x80x9cnear-surfacexe2x80x9d refers to a superficial zone having a depth in the range from about 0.025 mm (0.001xe2x80x3) to 0.5 mm (0.020xe2x80x3) for structural elements in the range from about 3 mm to 8 mm thick; for thicker elements up to about 25.4 mm thick the depth of the near-surface extends to a depth of about 0.75 mm (0.030xe2x80x3), the depth generally increasing proportional to the thickness. The xe2x80x9cnon-destructivexe2x80x9d test method, so termed because no damage is inflicted on the specimen, provided an indication of the level of residual stress at the surface where the measurement was made. Such detection was more effective than when, in the past, the likelihood of such a macroscopic defect was tested by stressing the specimen with a large, but not excessive force, sufficient to enlarge the defect but not destroy the specimen insofar as its intended use was concerned. There is no suggestion in the ""515 reference that tracking sequential changes in conductivity or impedance might be used to predict failure of the sample under continued stress.
U.S. Pat. No. 5,744,954 teaches a method of measuring physical properties in ferromagnetic metals by limiting the magnetic field strength of the test coils. It was recognized that a wide variety of physical properties in a metal object will be altered with heat treatment, for example, as well as resulting from fatigue or residual stress. It was also recognized that existing eddy current measurement techniques were incapable of detecting small conductivity changes even in so-called xe2x80x9cperfectxe2x80x9d metals lacking material or structural imperfections. Moreover, the ""954 patent contemplated using its measurement procedure to measure metal fatigue as well as physical condition after heat treatment, but offered no suggestion as to how this might be done.
U.S. Pat. No. 5,898,302 teaches a method of measuring residual stress in both ferromagnetic and non-ferromagnetic metals by measuring small imbalances in a bridge having four electrically identical induction coils which form a four terminal alternating current bridge circuit. Because the position of the coils on the metal surfaces is not changed, there is no xe2x80x9clift-offxe2x80x9d, and because the conductivities being measured are nearly the same, the difference measured is readily visible. This method detects very small changes in conductivity between a treated metal object which is worked sufficiently to produce a change in its metallurgical structure, which change is sensed and measurable as resistivity differences. These resistivity differences, attributable to the displacement of atoms or grains of the metal, are converted into residual stress. Different levels of residual stress may then be compared. Prior to the ""302 patent, the eddy current measurements detected relatively macroscopic defects not easily visible, or compositional variations in non-ferromagnetic metal objects. Using appropriate induction coils in the procedure taught in the ""302 patent, one can detect a change in conductivity due to stress corresponding to a change in impedance of the coils in the range from 0.01 to 0.001 percent, typically as small as 0.006 percent.
It is well known that stressing the near-surface of a metal article, for example by shot-peening the surface, provides protection to the extent that the residual compressive stress must be overcome by deformation of the surface before additional deformations cause the article to be so fatigued as eventually to fail. Typically the near-surface of an aluminum element from 3 mm to 6 mm thick is peened, or shot-peened to a depth in the range from about 0.127 mm (0.005xe2x80x3) to 0.381 mm (0.015xe2x80x3).
At the present time, structural elements such as aircraft and truck wheels, spars in the wing of an aircraft, brackets on locomotive wheels, and numerous other elements are surface-stressed for added protection. During the operating life of the structural element, e.g. an aluminum wheel, it is periodically inspected in annular zones known from experience to have a proclivity for failure. Such zones may be different for different wheels of different vehicles but typically include a first zone around the bolt holes, a second zone around the central opening in which an axle is held, and a third zone near where a tire""s bead is secured on the rim. As of this date, when a tire is demounted from the wheel for inspection of both, one or more of the annular zones is typically treated with a dye-penetrant and visually inspected. A visible indication of an incipient fissure, or worse, a crack, is cause to remove the wheel from service. However, either the incipient fissure or the crack is each an indication that failure due to fatigue has already occurred. The critical inspection was made too late because both, the additional protection provided by the shot-peened surface as well as that of the wheel after the added protection provided by shot-peening was used up, have been negated. With luck, the wheel is inspected and removed from further service before a catastrophic accident.
Methods of using eddy currents to compare precisely the electrical conductivity in a shot-peened surface to that of a similar, unpeened surface are disclosed in xe2x80x9cNon-destructive Residual Stress Measurement Using Eddy Currentxe2x80x9d, by Hong Chang, Fred C. Schoenig, and Jack A. Soules, Sixth International Conference on Shot-Peening, San Francisco, Calif., 1996; xe2x80x9cAdvances in Eddy Current Measurement of Residual Stressxe2x80x9d, by D. Barac, W. Katcher., and J. Soules, Seventh International Conference on Shot-Peening, Warsaw, Poland, 1999; and xe2x80x9cEddy Current Offers a Powerful Tool for Investigating Residual Stress and other Metallurgical Propertiesxe2x80x9d, by Hong Chang, Frederick C. Schoenig, Jr., and Jack A. Soules, Materials Evaluation, Vol. 57, No. 12, December 1999.
The accuracy of the foregoing methods allows one to observe the effect of repeated heavy loading on the surface conductivity of a metal article and dispenses with magnafluxing or using a dye penetrant.
Repeated loading of any metal article causes fatigue which, in turn, produces displacement of atoms, distortion of grain boundaries, work hardening and other metallurgical effects which may be sensed and quantified, for example by changes in X-ray diffraction patterns, or by changes in electrical conductivity. In particular, such effects are measurable as electrical conductivity. In a bent specimen, stress is largest in the near-surface where conductivity measurements may be made with precision.
Repeated cyclical loading of a metal structural element generates irregular and unpredictable variations of conductivity. Measurements of such variations cannot be used either to evaluate fatigue of a specimen being tested, or to predict its 50% failure point. However, by superimposing a superficial compressive stress in the near-surface of a zone prone to failure, for example, by working the near-surface of the specimen until it has a chosen level of residual stress, e.g. by peening, preferably shot-peening a portion of the specimen""s surface, and monitoring the conductivity (or resistivity) in that portion over a number of cycles of externally applied stress until the difference is about 50% to 60% of the difference between the initial resistivity at the chosen level of residual stress and a reference xe2x80x9cstandardxe2x80x9d, it is unexpectedly found that this difference is correlatable to the xe2x80x9csafe-operationxe2x80x9d point beyond which loading and unloading of the specimen progressively increases the risk of actual fatigue failure.
Assuming failure due to fatigue in an aluminum wheel of an aircraft""s landing gear is known to occur first in the third annular zone (the wheel""s rim near where the bead of the mounted tire is secured), by shot-peening a portion of the zone to a chosen extent, then measuring the difference in resistivity in that portion and a xe2x80x9cstandardxe2x80x9d sample, a xe2x80x9cbasexe2x80x9d value is obtained. After the wheel has been in service for a period during which the aircraft has been in service, the same shot-peened portion is measured to obtain the difference in resistivities between that portion and the xe2x80x9cstandardxe2x80x9d sample. By monitoring this difference over a prolonged period of operation of the aircraft, the imminent failure of the wheel may be predicted and the wheel removed from service at some chosen time when the difference in resistivities has reached a predetermined value, typically in the range from 50% to 60% of the original or base value.
In an analogous manner, a spar in an airplane wing is shot-peened in a zone known to be likely to fail first when the spar has been unduly fatigued, which zone is marked for easy identification. The difference in resistivities between that shot-peened portion and a xe2x80x9cstandardxe2x80x9d sample is monitored over the life of the aircraft to ensure that the difference in resistivities has not diminished more than 50% to relative to that difference between the peened portion at the beginning and the xe2x80x9cstandardxe2x80x9d sample.